The use of magnesium to reduce weight in automobiles has grown approximately 20% annually since the early 1990s. Most of this growth has been with interior component applications and, at the present time, the only magnesium powertrain components in production are nonstructural and in relatively low-temperature applications. Volkswagen used magnesium alloys AS41A and AS21 (Mg-4%Al, 1% Si and Mg-2% Al, 1% Si, respectively) in the 1970s to cast air-cooled engine blocks. Usage of these alloys ended when engine operating temperatures increased and the cost of magnesium increased. If the advantages of magnesium are to be extended to current engines and automatic transmission components, for example, several existing problems will have to be overcome.
Four issues for the use of magnesium permanent mold or die casting alloys in automotive powertrain components are: (1) creep (i.e., continued strain under stress), (2) cost, (3) castability and (4) corrosion. For example, the commercial die casting magnesium alloys (AZ91D, containing aluminum, zinc and manganese; AM60 and AM50, both containing aluminum and manganese) currently used in the automobile are limited to near-room-temperature applications because their mechanical properties decrease at higher temperatures and they are susceptible to creep at powertrain operating temperatures. AE42 is a rare earth element-containing magnesium die casting alloy (E designates mischmetal) that has creep resistance sufficient for automatic transmission operating temperatures (up to 150.degree. C.), but not engine temperatures (above 150.degree. C.).
Some magnesium alloys formulated for sand or permanent mold casting do provide good high-temperature properties and are used in aerospace and nuclear reactors. The high costs of exotic elements (Ag, Y, Zr and rare earths) used in these alloys prevent their use in automobiles.
Cost is also a major barrier to the consideration of magnesium for powertrain components. However, the cost differential between magnesium alloys and aluminum or iron is not as great as anticipated when costs are compared on an equal-volume basis. On a per pound basis, magnesium is significantly more expensive than iron and aluminum. However, when the density of the metals is considered and cost is adjusted to a per-unit volume basis, the cost differential is much less. Furthermore, using the costs of magnesium alloys that are sometimes projected, the differential per pound between magnesium and aluminum will be even less than the differential between aluminum and iron. Unfortunately, AE42 with its rare earth content is more expensive than the low-temperature magnesium alloys, so cost of high-temperature strength magnesium alloys remains an issue.
Castability has been an advantage of the current low-temperature magnesium alloys. These alloys are fluid and readily flow into and fill thin mold sections. In many of the non-powertrain applications, the conversion to Mg has enabled cost reduction by parts consolidation: casting complex parts rather than assembling many simpler parts. The excellent castability of these low-temperature magnesium alloys has also increased design flexibility and the use of thinner walls, both of which will be beneficial in powertrain components if the creep-resistant alloy has the same good castability. Unfortunately, AE42 and other proposed creep-resistant alloys do not have as good castability as AZ91D, AM60 and AM50. For example, some otherwise creep-resistant alloys tend to weld or seize to a metal die or their castings form cracks and must be rejected.
A fourth major concern for magnesium components is their corrosion behavior. This is because the powertrain components will be exposed to road conditions and salt spray. Corrosion has been overcome in the low-temperature alloys because their purity is carefully controlled and fastening techniques to prevent galvanic coupling have been established. Any powertrain alloy will need to have this same level of corrosion resistance.
Thus, one can project creep resistance, cost, castability and corrosion resistance as the key issues for a Mg alloy suitable for an internal combustion engine block or head or for a transmission case and then set requirements for the alloy that they will use, e.g.:
creep strength--20% greater than AE42 at 150.degree. C. PA1 cost, castability and corrosion resistance--equivalent to AZ91D
There remains a need for a magnesium alloy that can be forced into a die as a liquid, or poured into a permanent mold, and solidified to yield a casting that provides creep strength and corrosion resistance.